Polyethylene is divided into high-density (HDPE, density 0.941 g/cm3 or greater), medium-density (MDPE, density from 0.926 to 0.940 g/cm3), low-density (LDPE, density from 0.910 to 0.925 g/cm3), and linear low-density polyethylene (LLDPE, density from 0.910 to 0.925 g/cm3). See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials. Polyethylene can also be divided by molecular weight. For instance, ultra-high molecular weight polyethylene denotes those which have a weight average molecular weight (Mw) greater than 3,000,000. See U.S. Pat. No. 6,265,504. High molecular weight polyethylene usually denotes those which have an Mw from 130,000 to 1,000,000.
One of the main uses of polyethylene (HDPE, MDPE, LLDPE, and LDPE) is in film applications, such as grocery sacks, institutional and consumer can liners, merchandise bags, shipping sacks, food packaging films, multi-wall bag liners, produce bags, deli wraps, stretch wraps, and shrink wraps. The key physical properties of polyethylene film include tear strength, impact strength, tensile strength, stiffness and transparency. Film stiffness can be measured by modulus. Modulus is the resistance of the film to deformation under stress.
Machine direction orientation (MDO) is known to the polyolefin industry. When a polymer is strained under uniaxial stress, the orientation becomes aligned in the direction of pull. For instance, U.S. Pat. No. 6,391,411 teaches the MDO of high molecular weight (both Mn and Mw greater than 1,000,000) HDPE films. However, MDO of such high molecular weight HDPE films are limited because these films are difficult to stretch to a high drawdown ratio.
The current polyethylene films typically compromise several properties, such as modulus, yield strength, and break strength, to meet the package requirements for dart drop impact strength. Polymer films that do not compromise such properties are desirable for improving the performance of the bags, as well as the economics associated with producing and filling the bags. For example, by increasing the modulus and the yield strength of the film, larger bags can be produced, which would allow packaging larger quantities of goods while retaining their shape after being handled by the consumer. Bags with higher modulus would also allow the filling lines to run faster, improving the overall economics of the filling process.
By increasing the yield strength of the film, the bags would be less likely to elongate under stress and therefore they retain the original shape and dimensions. This would reduce the amount of breaks which are resulted from the film yielding and thinning under load. Also, the printed surface of the bag would not be distorted, maintaining the aesthetic quality of the package and enhancing brand recognition by the consumer.
In addition, the films that do not compromise the aforementioned properties could allow the reduction in the film thickness, further improving the economics associated with the products. Such innovations are desirable to all in the can liner and retailer bag industry for creating new products that provide both performance and economic benefit.